Nuts and Volts - May/June 2018

One of the things we learn with experience is that paper components work perfectly — it’s when they’re living in the real world that they can misbehave. The goal here was to use the DMX level to drive the number of LEDs that are on. As mentioned, I didn’t have access to the original code, so I speculated based on reports from the set.

NCO mode which is intended for PWM and pulse generation. Keep in mind that each cog has its own I/O configuration, and these get OR’d together for the physical pins; this lets us use one cog to monitor an I/O pin that is being controlled by another cog.

For the test, the PWM output is connected to P26. This lets me see changes on the LED and read the output and input values in the terminal window:

pub main | level, in
  setup

It seems like the original version divided the 0..255 range into eight thresholds and used that to select how many pixels were illuminated. The problem was on the ends: If zero is all off and 255 is all on, a little bit of noise in the pot on the lighting board could cause fluttering in the output (this was reported).

repeat
  repeat level from 0 to 255
 pwmo.duty(0, level)
 time.pause(50)
 
 term.rjdec(level, 3, “““)
 term.txn(“““, 4)
 in := 256 pwmtix / MS_005
 term.rjdec(in, 3, “““)

In my past, I’ve coded camera platform controls. Even very high quality joysticks do not center to the same place every time. In those devices, I used a dead band near the center so that a neutral joystick would cause both axes to stop. For this project, I used dead bands on the ends; that way, near zero would be full off, and near 255 would be full on.

term.tx(13)

Using 5% on either end gave me a dead band of 12 units, which means a full operational range of 232 units. Dividing 232 by eight (pixels), we get 29. It’s not a lot, but 29 brightness steps for each pixel is better than one.

Here’s the math for calculating pixel brightness from the DMX control inputs:

This is the loop that ramps the PWM signal output from 0 (0%) to 255 (100%).

After setting the output, there is a short delay to ensure that the background could measure the signal. The captured “on” ticks are converted to 0..255 with a simple equation. Easy peasy.

npix := (level - 12) / 29
partial := map((level - 12) // 29, 0, 28,
0, 255)
partial := partial brightness / 255

The next step was to fold the code into a standard object that allows both counters to be used and has the ability to measure the “high” or “low” ticks of a signal. We can also set the sample window period. With that in place, it was time to address the LED control for the costume piece.

Smooth Operator

Once everything was working, Rick asked if I could smooth out the behavior of the LEDs. In the original piece, the eight-pixel strip would turn on one pixel at a time — like a bar graph. It worked but there was a little jarring when a pixel would go from full off to full on. He asked if I could fade each pixel on instead, which he believed would result in a smoother looking animation. Of course, he was right.

The variable npix is the number of pixels that will be fully lit. This is calculated by subtracting 12 (dead band) from the control level, and then dividing by 29. The next line does two steps in one: It uses modulo (//) to determine the remaining steps in the last LED, then maps those steps to a normal brightness level of 0..255. The map() method scales an input range to an output range; in this case, we want to expand 0.. 28 to 0..255. The final line scales the partial brightness pixel to the brightness control channel value.

Rick and I finalized everything in his shop on Christmas day. Yeah, I know, working on Christmas day seems like a drag. It wasn’t really; he’s Jewish and I don’t have any family in Los Angeles — why not spend Christmas with one of my best friends making cool things in his shop?

Jon “JonnyMac” McPhalen jon@jonmcphalen.com The DMX receiver has four channels. The original piece used only one. Due to the nature of the show and a lot of nighttime action, the lighting folks asked if we could add an overall brightness control. Yes, we can, and that’s the variable called brightness (it’s the second value in the DMX stream).

Parallax, Inc. www.parallax.com

Propeller boards, chips, and programming tools Having been around movies and TV, we know that lighting situations can vary wildly. Knowing this and having spare cogs, we decided to add two more PWM input